The present invention relates to photoflash lamps and more particularly to high-voltage photoflash lamps.
A high-voltage flash lamp typically includes a glass envelope with a combustion-supporting gas and a quantity of filamentary, combustible material therein. A pair of electrically conductive lead wires is usually sealed in one end of the envelope and extend within the envelope. Medial portions of the extending ends of the lamp's conducting wires are located within a glass or ceramic bead. Primer material serves to bridge the portions of the ends which project through the bead. Flashing is accomplished by a firing pulse approaching a few thousand volts which is provided by a piezoelectric element. In another type of high-voltage lamp, the primer is located within an indentation in the bottom of the lamp and the conductive wires extend therein.
Understandably, it is highly desirous to prevent shred interference with the lamp's ignition. Shred interference can occur primarily in one of two ways: either by the shreds contacting and shorting the exposed portions of the lead wires within the envelope or by the shreds contacting and lying across the primer material surface.
In either case, the ignition voltage characteristics are altered, which in some instances can even prevent the lamp from firing. Shred interference can also reduce the firing voltage to the point that ignition is possible electrostatically. In situations wherein the lamp is used in circuitry containing several other lamps (e.g., sequential or random flash embodiments), an altered ignition voltage substantially reduces the lamp's compatibility with the desired circuit.
Various techniques for preventing shred interference with a lamp's ignition are illustrated. For example, in U.S. Pat. Nos. 3,884,615 and 3,685,947 a hollow glass bead is supported by the inner ends of the lead wires and primer is put into the cavity of the bead to electrically connect the two lead wires. This construction is bulky and does not permit miniaturization of the flash lamp. In addition, the beads are costly, and their mass, which is relatively isolated from the bulb, interferes with rapid cool down and liquification of oxygen, thereby limiting machine speeds.
In U.S. Pat. Nos. 3,823,994 and 3,627,459, the inner ends of the lead wires are exposed inside a small length of glass tubing which is sealed into the press. A quantity of primer within the tube connects the two lead wires electrically. This construction is costly in that it requires small pieces of fabricated glass tubing.
U.S. Pat. No. 4,229,161 teaches the use of a device, such as a disc of mica, to isolate electrically the shreds in the lamp from the primer. The beaded construction is also shown. Both the bead and the mica disc add considerably to the cost of the lamp and to the difficulty of cooling the lamp and thereby pressuring the lamp with oxygen.
It would be desirable to have a high-voltage photoflash lamp with improved reliability in which the shredded fill is insulated from the internal leads, the structure of which does not impede rapid cool down.